Except he wasn't. He had a down year with us thanks to Babcock, but he was still an important piece to have, and he still counts as value in the trade (as does Kerfoot, who you arbitrarily wrote off). The complaints about him were grossly exaggerated, and it was largely a result of people just not yet being able to accept the state of our goaltending. Barrie was the easy scapegoat.
We needed a cheap defenseman who could play top-four minutes, and we needed puck-moving ability from the right side of our defense, after it had been heavily exploited for multiple seasons and playoffs in a row. Barrie was a very good right-side defenseman with Colorado, coming off an amazing playoffs, and at 50% retained, he cost very little, and didn't require long-term investment that could negatively impact our cap, block future signings (like Brodie), or block our top prospects.
Even in hindsight, Barrie did have positives that he brought, especially over what we had previously had there, and he helped keep our head above water and in the playoff picture when the rest of the defense (and quite frankly, team) was obliterated by injury last year.
People tend to forget that we didn't know we had anything in Holl. We essentially had Rielly, Muzzin, Ceci, and Dermott as NHL defensemen, and Dermott was out to start the year and would be coming back off intensive shoulder surgery. That's it.
Kadri was coming off a pretty disappointing year, showing signs of decline, and had just been suspended for the 2nd playoffs in a row. And he vetoed the first trade. The value we got out of him was impressive at the time, and Kadri's impact was essentially replaced by Kerfoot at ES, and Nylander on the PP. And in the end, we get to enjoy Brodie (the original target), who is the exact type of defensemen everybody seemed to want out of the Kadri trade in the first place.
I liked Kadri while he was here, but the truth is, he's not an important enough piece to still be talking about, and he would not be putting up those point totals here, with the PP time he'd get here on a good day, even while being carried by Marner. That trade in no way comes close to the "worst trade in the last 20 years". That's a ridiculous statement. Even if we gave Kadri away for nothing, it wouldn't come close. He's a secondary piece. It's time to move on.